Process for the production of iron oxide containing catalysts

ABSTRACT

A process for the production mixed metal oxide containing catalysts comprising the steps of: dissolution of metals Me=Fe, Ni, Al, Cu, Co, Zn, Cr, in nitric acid providing an acid solution of metal mixed nitrate products, aluminium can be added either as nitrate or hydroxide; addition of a carbonhydrate, an amino acid and/or a carboxylic acid; 
         decomposition at 250-700° C. with free air supply of the acid solution by spraying onto the inner surface of one or more rotary kilns, into a spray calcination fluid bed, into a tower kiln or into a steel band conveyor furnace to iron oxide and NO x ; and optionally    regeneration of the formed NO x  to concentrated nitric acid and recycling of produced nitric acid to the first step.

The present invention relates to a process for the production of metaloxide being useful for the preparation of mixed metal oxide catalysts.

BACKGROUND OF THE INVENTION

The state of the art methods for preparing mixed metal oxide include useof process by precipitation. Metal sulphate is a cheap pure raw materialand good catalysts can be obtained by coprecipitation, but with a toohigh sulphur level. On the other hand, metal nitrate is an expensive rawmaterial, and dissolving metals in nitric acid require the expensivenitric acid.

The known methods for preparing mixed metal oxide catalysts are costlyin terms of purchasing the acid and basic raw materials and after theprecipitation, washings and waste-water treatment.

This is described in U.S. Pat. No. 4,482,645, where Jennings et al.prepare a solution of iron nitrate and chromium nitrate, to which sodiumcarbonate is added and the formed iron and chromium hydroxides arewashed before drying and decomposition to oxides.

The general object of this invention is, thus, to provide an improvedprocess for the production of mixed metal oxide containing catalysts bysimplified and inexpensive steps. Now an alternative manufacturing routebased on metals via nitrates and nitric acid recovery has been inventedand developed.

Compared to the known methods, the advantages of the invention include ahigh quality catalyst, and furthermore, high product yield throughreduced loss of material during the processing.

SUMMARY OF THE INVENTION

The process of this invention comprises the following steps involvingthe metals Me=Co, Zn, Fe, Ni, Cr and/or Cu:

(a) Dissolution of Me in nitric acid providing an acid solution of(exemplified by a valence 3 metal) Me(NO₃)₃ by reaction (i):2Me+8HNO₃→2 Me(NO₃)₃+2 NO+4H₂O

(b) Optionally mixing of different metal nitrate solutions, e.g.Fe(NO₃)₃ solution with Co(NO₃)₃ solution. Aluminium is added either asnitrate or hydroxide.

(c) Optionally, addition of promoters. If promoters (PR) as PR=Na, K,Cs, Rb, Mg, Ca, Ba, Sr are desired in the final product, they are addedas metal nitrates, carbonates, hydroxides etc to the dissolved Me(III)nitrate. The promoters are preferably added in the ratio range on molarbase <PR/Me<0.2.

The final solution will be Me(NO₃)₃ solution, optionally containingpromoters' PR nitrates.

The HNO₃ consumption will increase if NO₂ is formed.

(d) Thermal decomposition of the combined metal nitrate solution andpromoter acid nitrate solution into mixed metal oxide Me₂O₃ orhydroxyoxide MeOOH optionally containing the promoters as oxides ornitrates depending on the chemical nature of the promoter. NO_(x) gasesare also formed during the reaction. The reaction for the puredecomposition of Me(NO₃)₃ will be as reaction (ii):2Me(NO₃)₃→Me₂O₃+6NO₂+1.5O₂

(e) Optionally regeneration of NO_(x) gases from reaction (i) and (ii)in one or in a series of absorption towers to more or less concentratednitric acid according to reaction (iii):6NO₂+3 H₂O+1.5O₂→6HNO₃or for NO (iv)2NO+H₂O+1.5O₂→2HNO₃

Then the overall reaction for iron alone, i.e. when (i), (ii), (iii) and(iv) are combined, is:Total: 2Me+1.5O₂→Me₂O₃

When only the synthesis of the main component Me₂O₃ is considered, wesee that for the total reaction no by-product is formed and the otherraw material, oxygen, is taken from the air via the absorption towers.

The total reaction is somewhat influenced when promoters are included inthe mixed metal nitrate solution. The influence is dependent on whetherthe promoter PR is added as nitrate, hydroxide, oxide etc. If it forexample is added as KNO₃ with the molar ratio of K/Me=0.01 to totalreaction scheme will be:2Me+1.5O₂+0.02KNO₃+0.01H₂O→Me₂O₃3+0.01K₂O+0.02HNO₃.

This results in a slight formation of HNO₃ that can be used asdissolution.

NO and NO₂ or generally NO_(x) being formed in the above reactions (i)and (ii) is converted into nitric acid again in absorption towers.Reactions (iii) and (iv) result in formation of nitric acid that isrecycled and utilised for dissolution of Me, which is the main rawmaterial in the process.

Though, due to minor loss of the nitric acid, occasionally small amountsof nitric acid have to be added to the regenerated acid in order tomaintain or enhance dissolution of the raw material.

Decomposition in the above step (d) may be performed by spraying theacidic solution from step (a), (b) or (c) onto the inner surface of oneor more rotary kilns, into spray calcination fluid beds, into a steelband conveyor furnace or into a tower falling particles kiln with freesupply of air at 250-700° C. However, by using these methods, measuresmay be taken in order to prevent sticking of the product to the innersurface of the rotary kiln, e.g. by means of one or more sliding chains.

Adhesion of the prepared material from the decomposed acid solution tothe inner side of said furnaces or kilns might also be prevented byother physical or chemical means.

The metal oxide product is further improved by minor addition of anorganic compound capable of reducing nitrates. The reaction between theorganic compound and the nitrate will then generate a fasterdecomposition of the nitrates. Furthermore, the powder is weaklyagglomerated and possible to crush in low energy milling device.

The method of the invention is suitable for adding different additives(e.g. promoters in the case of catalyst, or other elements for otherpurposes) before decomposition. One way to decompose this solution tometal oxide(s) is continuously dripping the stock solution into a rotarykiln. The temperature in the rotary kiln may vary between 250-700° C.,preferable 350-600° C. An essential feature of the invention comprisesutilization of additives selected from organic compounds added to thenitrate stock solution. By adjusting additive quantity and/ortemperature in the rotary kiln it is possible to control powdercharacteristics (phase content/crystalline structure, surface area,particle size, microstructure etc). Without such organic additives, whenadding such an additive, the powder will freely run out from the rotarykiln making a continuous process possible. Preferred additives areselected from carbohydrates (glucose, fructose, lactose, sucrose orother saccharides), glycine and carboxylic acid. Moreover, the powder isagglomerated in hard and large lumps. Low quantity or none of theseadditives results always in α-Fe₂O₃. High quantity of an organicadditive results in γ-F₂O₃ when the pyrolysis temperature is low.

EXAMPLES Example 1

Iron was dissolved in nitric acid together with nitrates of thepromoters such as Cr, Cu, K and Na in the required proportions and astock solution was obtained. This solution was dripped at given feedrate into rotary kiln at 350° C. Powder characteristics were measuredusing XRD analysis and isothermal nitrogen adsorption for specificsurface area (according to Brunauer, Emmett and Teller theory). Surfacearea on synthesised powder (measured by nitrogen adsorption) was 73m²/g.

Disadvantage: The powder was strong adhered to the rotary kiln walls andvery hard particles.

Example 2

The stock solution was prepared similar to Example 1. Then a certainquantity of glucose was dissolved corresponding to the ⅙ of theso-called “stoichiometric ratio” between oxidising (nitrates) andreducing (glucose) reactants. This solution was dripped at given feedrate into a rotary kiln at 400° C.

X-ray analysis resulted in α-Fe₂O₃ with parameters a=5.035 Å, c=13.758 Åand crystallite size D⁽⁰²⁴⁾=285 Å.

Surface area on synthesised powder (measured by nitrogen adsorption) wasabout 50 m²/g.

SEM investigation showed a unique microstructure consisting of largeporosity with cavities up to 5 μm. Using atomic-resolution TEM, ahomogeneous distribution of Fe, Cu, and Cr was found. In the same timevery unusual crystal morphology was observed by TEM examination.

Example 3

The stock solution was prepared similar to Example 1. Then a quantity ofglucose was dissolved corresponding to a ratio greater than ½ of theso-called “stoichiometric ratio” between oxidising (nitrates) andreducing (glucose) reactants. This solution was dripped at given feedrate into rotary kiln at 350° C.

X-ray analysis resulted in a cubic γ-Fe₂O₃ with parameter a=8.333 Å, andcrystallite size D⁽⁴⁴⁰⁾=97 Å.

Surface area on synthesised powder (measured by nitrogen adsorption) was70 m²/g.

Such powders are used in magnetic tape memories.

Example 4

The stock solution was prepared similar to Example 1. Then a certainquantity of glucose was dissolved corresponding to the ½ of theso-called “stoichiometric ratio” between oxidising (nitrates) andreducing (glucose) reactants. This solution is dripped at given feedrate into rotary kiln at 400° C.

X-ray analysis resulted in α-Fe₂O₃ and γ-Fe₂O₃.

Example 5

A mixture of metallic Co and Fe in the molar ratio 1:1 is dissolved inconcentrated nitric acid. Aluminium hydroxide is added to an overallmolar ratio of Fe:Co:Al of 1:1:2. Finally KNO₃ is added so the molarpercentage is 1%.

This solution is dripped at a constant feed rate into a rotating kiln at350° C.

The powder is crushed, sieved, mixed with graphite and pelletised.

The pellets are calcinated in a conveyor furnace at a temperature of550° C.

The pellets are reduced in pure hydrogen and are useful as catalyst forboth ammonia synthesis and decomposition.

Example 6

A mixture of metallic Cu and Zinc oxide, ZnO, is dissolved inconcentrated HNO₃. The molar ratio is 1:1. Alumina is added to anoverall molar ratio of Cu:Zn:Al of 1:1:1. The slurry is decomposed at350° C.

The powder is crushed, sieved, mixed with graphite and pelletised.

The pellets are reduced in dilute hydrogen and are useful as catalystfor methanol synthesis or WGS conversion.

1-6. (canceled)
 7. A process for the production of a mixed oxidecatalyst with iron oxide and metal oxides selected from one or more ofthe oxides of Co, Al, Ni, Zn, Cu and Cr, the process comprising thesteps of: (a) providing a nitric acid solution with iron nitrate and anitrate of one or more metals of Co, Al, Ni, Zn, Cu and Cr; (b) addingto the nitric acid solution a carbohydrate; and (c) decomposing thesolution obtained in step (b) at a temperature of between 250 and 700°C. with free air supply to obtain the mixed oxide catalyst containingthe iron oxide in its alpha and/or gamma form.
 8. A process according toclaim 7, wherein the carbohydrate is chosen from glucose, fructose,lactose or sucrose.
 9. A process according to claim 8, wherein thedecomposing takes place at 300-450° C.
 10. A process according to claim8, wherein the amount of glucose being added to the solution of step (b)is between 0.05 to 0.3 mol per iron nitrate mol.
 11. A mixed metal oxidecontaining catalyst produced by a process according to claim 7.